1. Technical Field
The present disclosure relates to a suspension control device for a vehicle, e.g., a motor vehicle.
2. Description of the Related Art
As a suspension of a vehicle, there is known a suspension including an actuator for controlling an up/down stroke of the suspension or a force applied to the suspension. As a control device for this type of the suspension, there is known a control device for predicting an undulation of a road surface ahead of the vehicle, estimating a displacement in a vertical direction of the road surface based on a result of the prediction, and controlling the actuator so that the up/down stroke of the suspension changes depending on the displacement in the vertical direction of the road surface.
For example, in Japanese Patent Application Laid-open No. Hei 4-342612, there is described a control device configured to estimate a relative displacement between a vehicle body and a road surface and an up/down acceleration of the vehicle body at a prediction position ahead of a front wheel by a prediction distance, and to output to an actuator a command value for an optimal force or stroke when the front wheel reaches the prediction position based on the estimations. With this type of the control device, for example, when the relative displacement between the vehicle body and the road surface and the up/down acceleration of the vehicle body are detected, and the actuator is controlled based on a result of the detection thereof, a delay in control can be decreased, thereby decreasing a force applied from the road surface to the vehicle body via the front wheel.
With the control device disclosed in Japanese Patent Application Laid-open No. Hei 4-342612, when the change in the up/down displacement of the road surface is continuous, and a change rate of the up/down displacement is not high, a position of the wheel in the up/down direction can be changed depending on the up/down displacement of the road surface, thereby effectively decreasing the force applied from the road surface to the vehicle body via the front wheel.
However, on a portion where the change in the up/down displacement of the road surface is discontinuous or a portion where the change rate of the up/down displacement of the road surface is high, such as a step that the vehicle needs to move over and pass, a change amount of the relative displacement between the vehicle body and the road surface and a change amount of the up/down acceleration of the vehicle body are high at the prediction position. Therefore, with the control device described in Japanese Patent Application Laid-open No. Hei 4-342612, the command value for the force or the stroke output to the actuator rapidly fluctuates, and the force applied from the road surface to the vehicle body via the front wheel cannot effectively be decreased.
When there exists roughness on the road surface, such as a step whose change in the up/down displacement is discontinuous and large, a state occurs where the wheel comes into contact with two portions of the road surface viewed in a direction along a rotational axis of the wheel, and a portion on a leading side in a travel direction of the vehicle out of the two portions is higher than the other portion. When the vehicle passes over this roughness, the actuator is controlled so that the command value for the force or the stroke output to the actuator does not rapidly fluctuate. For example, as illustrated in FIG. 12, when a wheel 200 comes into contact with a road surface 202 at two portions P1 and P2, the actuator is controlled so that the lowest point P0 of the wheel 200 moves along a target trajectory 204 connecting the two portions P1 and P2 of the road surface 202 with each other.
However, as indicated by the broken line of FIG. 12, when the lowest point P0 of the wheel 200 is between the two portions P1 and P2, the wheel 200 moves toward the travel direction of the vehicle while pressing the portion P2. Thus, the wheel 200 receives a reaction force Fr of a pressing force Fp from the portion P2, the reaction force Fr acts on the wheel 200, and this force is transmitted from the wheel 200 to the vehicle body, which is not shown in FIG. 12. Thus, the force applied from the road surface to the vehicle body via the wheel and a vibration of the vehicle body caused by this force cannot effectively be decreased.